Back pack signal light unit



FIG. 1 is a perspective view of a back pack signal light unit, showing my new design;

FIG. 2 is an end elevational view thereof, as taken from the left hand end of FIG. 1;

FIG. 3 is an underside view thereof; and

FIG. 4 is a rear elevational view thereof. 

The ornamental design for a back pack signal light unit, substantially as shown and described. 